The invention relates to electronic devices, and, more particularly, to junction field effect transistor drivers.
Electronic components, such as integrated circuits and displays, typically have power requirements which differ from the primary power supply characteristics. For example, portable computers may contain integrated circuits operating with a 3.3 volt DC supply and a backlit display screen operating at 1500 volts but the primary power consists of rechargeable batteries whose output voltage at full charge may be 5 volts and which drops exponentially as the batteries are discharged. Hence, electronic systems typically will include power supplies with AC-DC converters, DC-DC regulators, or DC-AC inverters to provide output power with the required characteristics.
DC-DC regulators most commonly utlilize switching regulation and may use a push-pull arrangement as illustrated in FIG. 1a with waveforms shown in FIG. 1b. Basically, the pulse width modulated (PWM) driver alternately switches on and off power devices Q1 and Q2 to excite the transformer primary, and the transformer secondary feeds a self-commutating synchronous rectifier followed by an LC filter for output. The synchronous rectifier uses n-channel MOSFETs rather than diodes; this avoids the turn on voltage drop of diodes which can be significant for low output voltage power supplies. Resistor divider R senses the output voltage and feeds this back to the PWM driver. If the output voltage is too low, then the PWM driver increases the duty cycles of Q1 and Q2, and conversely, if the output voltage is too high, the duty cycles of Q1 and Q2 are reduced. The PWM driver may simply be an error amplifier (amplifying the difference between desired and actual output voltages) feeding one input of a comparator with a sawtooth voltage having a fixed frequency feeding the other comparator input; the comparator output would be the input signal for a driver for power device Q1 and a similar phase-shifted comparator would be the input signal for a driver for power device Q2. FIG. 1c illustrates such a possible half of a PWM driver.
Switching power supplies for portable computers and other portable electronic equipment generally benefit from higher switching frequencies because the size and weight of the magnetic portions (transformers and inductors) can be reduced. And the trend towards lower operating voltages for integrated circuits to reduce power consumption requires the output rectifiers of related power supplies have low on resistance and minimal voltage drop.
Junction field effect transistors (JFETs) typically are depletion mode devices with n-type channels and require a negative gate voltage to pinch off the channel and turn off. Thus the driver for a JFET also requires a negative power supply. Further, injecting carriers from the gate of a JFET into the channel while the JFET is turned on can lower the channel and drift region resistance (R.sub.ON) and thereby minimizes ohmic losses. This "bipolar mode" of operation requires a small positive voltage to forward bias the gate, and so the JFET driver would further require a small positive power supply. See Baliga, Modern Power Devices pp.175-182 (Krieger Publ., Malabar, Fla. 1992). Thus the known drivers for JFET devices have problems including inefficient circuitry.
JFETs made be made of gallium arsenide to lower R.sub.ON due to the higher electron mobility of gallium arsenide as compared to silicon. JFETs frqeuently have a vertical channel structure and thus may be called VFETs. FIGS. 2a-b heuristically illustrate in perspective and cross sectional elevation views VFET 100 as including a source 102, multifinger gate 104, channel region 106 between the gate fingers, drain 108, source contact 112, gate contact 114, and drain contact 118. U.S. Pat. No. 5,231,037 describes a method of fabrication for such VFETs.